1. Field of the Invention
The present invention relates to a group-3 nitride semiconductor device (referred to as device hereinbelow), and in particular a nitride semiconductor light emitting device and a manufacturing method thereof.
2. Background Art
Extensive research is now underway on gallium nitride (referred to as GaN hereinbelow) and related compounds as a material system for a shortwave light emitting device, in particular, a shortwave semiconductor laser. A GaN-based semiconductor laser device is manufactured by successively depositing semiconductor single-crystal layers such as (Alx Ga1xe2x88x92x)1xe2x88x92yINyN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61) on a crystal substrate.
A metal organic chemical vapor deposition method (abbreviated as MOCVD hereinbelow) is generally used to produce such a single-crystal layer. In this method, source gases containing trimethyl gallium (abbreviated as TMG hereinbelow) as a group-3 precursor material and ammonia (NH3) as a group-5 precursor material are introduced into a reactor to react at a temperature within the range of 900-1000xc2x0 C., thereby depositing compound crystals on the substrate. A multi-layer structure comprising various compounds can be obtained by changing the ratio of the precursors fed into the reactor to grow many different layers on the substrate.
If the deposited single-crystal layer has many penetrating defects, the light emitting performance of the device is deteriorated substantially. Such defect is called threading dislocation, which is a linearly extending defect that penetrates the crystal layer along the growth direction. Since a threading dislocation acts as a non-radiative recombination center for carriers, a semiconductor light emitting device comprising a layer with many dislocations suffers from poor luminous efficiency. The above mentioned defect is generated due to crystal misfit strain at an interface between the substrate and an overlying layer formed thereon. Attempts to reduce the effect of the misfit at the interface have been made by choosing substrate materials having similar crystal structure, lattice constant, and thermal expansion coefficient to those of GaN-based crystal.
A material, which meets the above requirements and has good compatibility with a substrate, is a semiconductor crystal itself. However, as for group-3 nitride semiconductors (AlxGa1xe2x88x92x)1xe2x88x92yInyN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61), it is inevitable to use dissimilar materials such as sapphire or the like, because there is no nitride semiconductor bulk crystal which is most suitable for a substrate. Sapphire has a lattice constant different from that of GaN by about 14%.
On approach, known as the two-step-growth method, was proposed to accommodate the misfit at the interface between a sapphire substrate and a GaN-based single-crystal layer grown thereon to reduce the generation of crystal defects in the GaN-based single-crystal layer. This method comprises the steps of forming a lower-temperature buffer layer consisting of aluminum nitride (AlN) on the sapphire substrate at a lower temperature within the range of 400-600xc2x0 C., and then forming a GaN single-crystal layer over the lower-temperature buffer layer. However, the above method has not been completely successful in reducing the generation of such defects that pass through the GaN single-crystal layer.
Generally, as a dislocation in semiconductor crystals acts as a non-radiative recombination center and is substantially responsible for degrading the light emitting characteristics of light emitting devices such as light emitting diodes and semiconductor lasers, it is desirable that the crystals in these devices do not includes any dislocations. Therefore, research is now underway toward reduction of threading dislocations.
A main object of the invention is to provide a nitride semiconductor light emitting device having good luminescent characteristics.
Another object of the invention is to provide a method for manufacturing the above nitride semiconductor light emitting device whereby the generation of defects passing through a single-crystal layer formed on a substrate can be reduced.
The nitride semiconductor light emitting device according to the present invention comprises an active layer comprising group-3 nitride semiconductors, and a barrier layer made of a predetermined material and provided adjacent to the active layer. The barrier layer has a greater bandgap than that of the active layer. The light emitting device further comprises a barrier portion, or buried barrier portion defined by interfaces surrounding a threading dislocation in the active layer made of the same material as the barrier layer.
The nitride semiconductor light emitting device according to the present invention has a feature in that the active layer has one of a single and multiple quantum well structure.
The nitride semiconductor light emitting device according to the present invention has a feature in that the predetermined material of the barrier layer fills up a recess enclosed with the interfaces on the active layer to smooth surfaces of the recess.
The nitride semiconductor light emitting device according to the present invention has a feature in that the barrier portion has one of a cone-shape, truncated cone shape, and combination thereof.
The nitride semiconductor light emitting device according to the present invention has a feature in that the group-3 nitride semiconductor single-crystal layers are of (AlxGa1xe2x88x92x)1xe2x88x92yInyN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61).
The nitride semiconductor light emitting device according to the present invention further comprises a low temperature barrier layer provided between the barrier layer and the active layer, and that the low temperature barrier layer is formed of substantially the same predetermined material as that of the barrier layer at substantially the same temperature as the growth temperature of the active layer.
The nitride semiconductor light emitting device according to the present invention has a feature in that the low temperature barrier layer has a lower AlN composition ratio than that of the barrier layer.
According to the present invention, in order to provide a nitride semiconductor light emitting device comprising an active layer provided by depositing group-3 nitride semiconductor single-crystal layers (AlxGa1xe2x88x92x)1xe2x88x92yInyN (0xe2x89xa6x1, 0xe2x89xa6yxe2x89xa61) and a barrier layer provided adjacent to the active layer with a greater bandgap than that of the active layer, a method comprises the steps of forming a pit defining a recess attributable to a threading dislocation in semiconductor layers formed on a substrate in the active layer of group-3 nitride semiconductors, and depositing a material of the barrier layer onto the active layer to form a barrier portion surrounding the threading dislocation and having an interface defined by the side surface of the recess.
The method according to the present invention has a feature in that the step of forming the pit comprises a step of etching the active layer after the active layer is formed.
The method according to the present invention has a feature in that the etching in the step of etching is terminated when erosion along the threading dislocation partially reaches the underlying semiconductor layer.
The method according to the present invention has a feature in that the step of forming the pit comprises the step of forming the semiconductor layer at a temperature within a range of 600-850xc2x0 C. prior to the growth of the active layer.
The method according to the present invention has a feature in that the method further comprises the step of forming a low temperature barrier layer of substantially the same material as that of the barrier layer at substantially the same temperature as a growth temperature of the active layer between the step of forming the pit and the step of depositing the material.
The method according to the present invention has a feature in that the low temperature barrier layer has a lower AlN composition ratio than that of the barrier layer.
According to the present invention, epitaxial growth of a wafer includes a step of forming pits in the active layer either by etching the wafer after the growth of the active layer is finished, or by growing a part of the guiding layer at lower temperature prior to the growth of the active layer. The epitaxial growth further includes a step of burying the pits which are defined by the interfaces surrounding the threading dislocations extending into the active layer, with a material having a wider bandgap than that of the active layer. Described above, the growth for a device wafer is completed.
According to the present invention, an barrier portion having a wider bandgap than that of an active layer surrounds a threading dislocation, thereby preventing the diffusion of carriers to the dislocation so that a device has the improved light emitting characteristics.
According to the present invention, an barrier portion having a greater bandgap than that of an active layer surround a threading dislocation, in the vicinity of which a carrier does not spread, so that a device has the improved light emitting characteristics.